Prior to the beginning of the 1970s mainly graphite electrodes were used as anodes in electrochemical processes. Graphite anodes have a number of advantages, namely: an inexpensive and readily-available electrode material is used, the anodes are insensitive towards short-circuits. At the same time, however, graphite anodes are catalytically less active which necessitates a high voltage on the electrolyzer, they also have a high abrasion degree thus causing a frequent disassembling of the electrolyzer for replacement of anode sets. Furthermore, graphite anodes have high overall dimensions and weight which results in an undesirable increase of the electrolyzer size, as well as floor space of the electrolysis shops.
At the present time in electrochemical processes metal-oxide anodes which comprise a current-conducting support with an active coating of metal oxides deposited thereon are extensively used. The current-conducting support is made of a metal passivated by anodic polarization, for example, titanium, tantalum, zirconium, niobium, or an alloy of these metals. The current-conducting support or base can be made of any shape, for example in the form of a flat plate with or without perforation, or in the form of a rod, screen, grid, a metal-ceramic body.
Known in the art is an electrode for electrochemical processes comprising a current-conducting support, for example, of titanium with an active coating of a mixture of oxides of titanium, iridium and ruthenium applied thereon. The content of titanium oxides in this coating is considerable and amounts to more than 75 molar percent. For example, in such prior art electrode, the coating has the following composition, molar percent: IrO.sub.2 --3.91, RuO.sub.2 --7.43 and TiO.sub.2 --88.66 (samples B and D taught in U.S. Pat. No. 3,948,751, Table 2); IrO.sub.2 --7.25, RuO.sub.2 --13.8 and TiO.sub.2 --78.95 (sample C, U.S. Pat. No. 3,948,751, Table 2).
As it is clear from the above proportions, the coating ingredients in the above electrode are present in the following ratio (mol.%): TiO.sub.2 (IrO.sub.2 +RuO.sub.2)=(3.8-7.8):1 and IrO.sub.2 : RuO.sub.2 =0.5:1 (cf. U.S. Pat. No. 3,948,751, Cl. 204-290F., 1976).
This prior art electrode features instability of the anode potential in electrolysis, a low current yield and an insufficient resistance of the electrode in operation.
Thus, in testing samples "B", "C" and "D" in a saturated NaCl solution at the current density of 1 A/cm.sup.2 and temperature of 65.degree. C. the anode potential of sample "B" varied within the range of from 1.53 to 1.62 V during 2,000 hours of electrolysis (about 83 days); the anode potential of sample "C"--within the range of from 1.35 to 1.38 V during 2,300 hours (about 96 days) of electrolysis and that of sample "D"--within the range of from 1.44 to 1.50 V during 816 hours (34 days) of electrolysis.
The most extensive use has been enjoyed by metal-oxide anodes with an active coating containing 30 mol.% of ruthenium dioxide and 70 mol.% of titanium dioxide known in the art as DSA (dimensionally-stable anodes) and in the USSR available under the registered trademark OPTA. The OPTA electrodes are protected by the USSR Inventor's Certificate No. 369923.
The rate of consumption of the active mass of this electrode under stationary chlorine electrolysis conditions at a current density of 0.2 to 0.4 A/cm.sup.2, as determined by the radiochemical method, is equal to 2.6.times.10.sup.-8 g/cm.sup.2.hr.
The resistance of the active coating of an electrode can be determined by the method of variable polarity and amalgamation which is widely used as a rapid method for the evaluation of quality of an active coating: amalgamation resistance, adhesion to the current-conducting support, resistance to cathodic polarization and shortcircuit resistance.
The results of measurements of consumption of the active coating mass of an OPTA electrode obtained by the method of variable polarity and amalgamation are shown in the following Table.
______________________________________ Number of testing cycles 1-3 4-6 7-9 10-12 13-15 16-18 ______________________________________ Consumption of 0.595 0.610 0.140 0.180 0.190 0.170 the active mass for every 3 suc- cessive testing cycles, mg/cm.sup.2 ______________________________________
An OPTa electrode, as compared to a graphite one, under conditions of electrolysis of chlorides of alkali metals makes it possible to lower overtension of chlorine evolution, to reduce voltage on the electrolyzer and to save about 200 kW.hr per ton of caustic soda (as calculated for a 100% product), as well as to improve purity of the electrolysis products, extend the service life of anodes from 7-8 months to 5-7 years and reduce costs of electrode reassembling. However, an OPTA electrode has the following disadvantages: a relatively high rate of consumption of noble metals especially noticeable in a mass-scale use of such electrodes; an insufficient resistance of the coating under conditions of a combined evolution of oxygen and chlorine; at an increased content of oxygen in the anodic gas a "closing" of electrode occurs at a still high residual content of ruthenium in the active coating of the electrode. These factors impair the operation reliability of this electrode, especially under conditions of electrolysis with an ion-exchange membrane.